As is generally known in the art, the unavailability of a link between a hybrid digital terminal (HDT), also known as a host digital terminal, and a network interface unit (NIU), also known as remote service units (RSUs), is conventionally assessed using parameters such as Mean Time to Repair (MTTR) and Mean Time Between Outages (MTBO). With these parameters, it is possible to calculate the usual link unavailability per year. Although these parameters are useful and have yielded acceptable results, using these parameters does not circumvent the inherent uncertainties associated with the measurement of the time between system failures and the time taken to recover from system defects.
Other methods of determining system failures in a network have been described in such U.S. Patents as U.S. Pat. No. 5,553,059, issued to Emerson et al on Sep. 3, 1996, and entitled, "Network Interface Unit Remote Test Pattern Generation;" U.S. Pat. No. 4,451,827, issued to Kahn et al on May 29, 1984, and entitled, "Local Area Communication Network;" and U.S. Pat. No. 5,566,161, issued to Hartmann et al on Oct. 15, 1996, and entitled, "Adaptive DS1 Frame Format Conversion Device for Remotely Monitoring and Testing the Performance of Telephone Circuits." However, all of these methods require additional hardware to implement the testing of the links between the various devices. For example, Emerson et al teach a system for isolating the direction of communication errors in the local loop of a digital data network. The system of Emerson et al uses a NIU loopback test to isolate transmission problems at a customer's premises from problems on the local loop. To do this, Emerson et al add a test pattern generation capability to the NIU, which allows the remote test system to determine whether the transmission problem on the local loop is in the network-to-customer premises direction or in the customer premises-to-network direction. As another example, Hartmann et al teach another modified NIU for remotely monitoring and testing the performance of DS1 telephone circuits, which are installed on the network side of an interface between customer premises equipment and equipment provided by the network provider. The improved NIU of Hartmann et al is used to non-intrusively collect and transmit full-time performance monitoring data to the network provider. Essentially, the NIU device of Hartmann et al provides this non-intrusive monitoring of the customer premises equipment by performing an adaptive real-time DS1 circuit frame format conversion, thus facilitating the conversion of the customer premises equipment generated signal frame formats to the Extended Superframe Format according to the ANSI T1.403 Standard with Performance Report Message.
However, all of these methods of assessing the unavailability of a link between an NIU and a HDT require significant hardware modifications to the NIU or to the network as a whole. This is very costly and therefore, there exists a need in the art to provide a method to assess whether a link has failed without the need for substantial additional hardware. The present invention addresses such a need.